Interest in fuel cell batteries as power sources for portable electronic devices has grown. A fuel cell is an electrochemical cell that uses materials from outside the cell as the active materials for the positive and negative electrode. Because a fuel cell does not have to contain all of the active materials used to generate electricity, the fuel cell can be made with a small volume relative to the amount of electrical energy produced compared to other types of batteries.
Fuel cells can be categorized according to the types of materials used in the positive electrode (cathode) and negative electrode (anode) reactions. One category of fuel cell is a hydrogen fuel cell using hydrogen as the negative electrode active material and oxygen as the positive electrode active material. When such a fuel cell is discharged, hydrogen is oxidized at the negative electrode to produce hydrogen ions and electrons. The hydrogen ions pass through an electrically nonconductive, ion permeable separator and the electrons pass through an external circuit to the positive electrode, where oxygen is reduced.
In some types of hydrogen fuel cells, hydrogen is formed from a fuel supplied to the negative electrode side of the fuel cell. In other types of hydrogen fuel cells, hydrogen gas is supplied to the fuel cell from a source outside the fuel cell. A fuel cell system can include a fuel cell battery, including one or more fuel cells, and a hydrogen source, such as a fuel tank, a hydrogen tank or a hydrogen generator. In some fuel cell systems, the hydrogen source can be replaced after the hydrogen is depleted. Replaceable hydrogen sources can be rechargeable or disposable.
A hydrogen generator uses one or more reactants containing hydrogen that can react to produce hydrogen gas. The reaction can be initiated in various ways, such as hydrolysis and thermolysis. For example, two reactants can produce hydrogen and byproducts. An accelerator and/or a catalyst can be used to increase the rate of reaction or catalyze the reaction. When the reactants react, reaction products including hydrogen gas and byproducts are produced.
Some types of hydrogen generators include a first reactant in solid form and a second reactant in fluid form. The first reactant can be formed into one or more solid forms, referred to herein as pellets. The first and second reactants are initially separated, and the second reactant is transported to come in contact with the first reactant, and the reactants react to produce hydrogen gas. Transport of the fluid can be controlled so that hydrogen is produced as need by an external device such as a fuel cell stack.
It is desirable for the first and second reactants to react efficiently and completely to provide the maximum quantity of hydrogen for a given hydrogen generator size. To accomplish this, good contact is required between the first and second reactants throughout the use of the hydrogen generator. Accumulation of byproducts around unconsumed portions of the pellet and accumulation (e.g., pooling) of fluid can both interfere with good contact between unreacted first and second reactants. Prior attempts have been made to improve the contact between the first and second reactants by providing good distribution of fluid to the pellet and removing byproducts from the vicinity of unconsumed portions of the pellet. Examples of such attempts are disclosed in U.S. Pat. Nos. 3,820,956 and 7,097,813, and in US Patent Publication Nos. 2008/0216906, 2009/0274595, 2008/0014481 and 2009/0104481. However, prior hydrogen generators have had a variety of shortcomings, and further improvement is desirable.
In view of the above, it is desirable to provide a hydrogen gas generator that can produce a maximum amount of hydrogen per unit volume of the hydrogen generator. Excellent reaction efficiency, with maximum use of reactants is desired, as is utilization of the internal volume of the hydrogen generator for all components. It is also desirable for the hydrogen generator to have a simple design, be easily manufactured and have a low cost.